In a metal-oxide-semiconductor (MOS) transistor such as a MOS field-effect-transistor (MOSFET), electric current flows through a surface region under a gate electrode and a gate oxide when an electric field is applied to the source and drain junction regions while a gate charge is applied. The surface region through which the electric current flows is known as a channel. The characteristics of a MOSFET are determined by a dopant concentration in the channel. More specifically, it is very important to precisely dope impurities into the channel region because device characteristics such as the threshold voltage of the transistor and the drain current are subject to the dopant concentration.
Conventional channel doping is achieved by performing well ion implantation, channel ion implantation, or threshold ion implantation. Channel structures formed by such ion implantation include a flat channel in which a dopant concentration is uniform through the whole region of the channel, a buried channel which is formed at a predetermined distance from the top surface of a semiconductor substrate, and a retrograde channel which has a vertically increasing doping profile from the top surface of the channel. Retrograde channels are widely used for high performance microprocessors requiring a channel length less than 0.2 μm. In such a context, the retrograde channel is generally formed by heavy ion implantation using indium (In), arsenic (As), or antimony (Sb). The retrograde channel is suitable for high performance MOSFET devices with high driving current characteristics because a low dopant concentration in its surface increases the surface mobility of an electric current.
As the degree of integration of a semiconductor device increase, the channel length is shortened, and a very thin channel is required. However, conventional ion implantation technology cannot achieve a retrograde channel less than 50 nm in depth. To solve such a problem, an epitaxial channel has been suggested. However, the epitaxial channel has not achieved an improvement in the current on-off characteristics because it is difficult to control the loss and diffusion of the channel dopants due to an epitaxial layer formation process and a later thermal treatment process.
The most ideal channel doping method may embody a δ-doped epitaxial channel. However, according to the reported findings, both doped and undoped epitaxial layers cannot be made into a δ-doped epitaxial channel less than 30 nm in depth because of later dopant diffusion.
To solve such a problem, a method for preventing diffusion of dopants in a δ-doped layer has been suggested in Lee and Lee, Laser Thermal Annealed SSR Well Prior to Epi-Channel Growth (LASPE) for 70 nm nFET, IEDM 2000. The suggested method performs channel doping by using an ultra-low energy ion implantation and an instant laser annealing. According to the suggested method, the instant laser annealing controls the diffusion and loss of dopants during a selective epitaxial growth.
However, the laser power for the laser annealing may cause partial melting of the silicon substrate surface, thereby deteriorating the surface roughness and causing crystal defects. Therefore, the laser annealing method is not applicable to practical semiconductor device manufacturing processes.
FIG. 1 is a cross-sectional view of a conventional transistor having a super steep retrograde (SSR) epitaxial channel. Although conventional transistor fabrication technology has reduced the depth of the channel by forming a retrograde channel 7 as shown in FIG. 1, it has failed to substantially reduce the length of the channel.